Backscatter reduction in thin electron detectors

ABSTRACT

In a direct electron detector, backscattering of electrons into the detector volume from below the sensor is prevented. In some embodiments, an empty space is maintained below the sensor. In other embodiments, a structure below the sensor includes geometry, such as multiple high aspects ratio channels, either extending to or from the sensor to trap electrons, or a structure of angled surfaces to deflect the electrons that pass through the sensor.

This application is a Continuation of U.S. patent application Ser. No.13/197,632, filed Aug. 3, 2011, which claims priority from U.S.Provisional Patent Application 61/370,737, filed Aug. 4, 2010, all ofwhich are hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to improving resolution in electrondetectors for transmission electron microscopy.

BACKGROUND OF THE INVENTION

In the early '90s, CMOS Monolithic Active Pixel Sensors (MAPS) wereinvented for the detection of visible light. Because of continuousimprovements in CMOS technology, CMOS sensors are becoming the dominantimage-sensing device, in commercial digital cameras and scientificapplications.

CMOS MAPS were also proposed and demonstrated as charged particledetectors, first for particle physics, and then for other applications,such as transmission electron microscopy. The electron energies in atransmission electron microscope (“TEM”) typically range from about 100keV up to about 500 keV. TEMs commonly use charged-couple device (“CCD”)detectors, which are damaged by high energy electrons. To prevent damageto the CCD detector, TEM detectors include a scintillator that convertsthe electrons to light, which is then detected by the CCD. Theintervening scintillator reduces resolution of the detector. CMOS MAPScan be used as direct detectors of charged particles, that is, the CMOSMAPS are more robust and can detect the electrons directly.

CMOS MAPS can provide a good signal-to-noise ratio, high resolution andhigh sensitivity, and are a significant improvement over current CCDtechnology using scintillators. CMOS MAPS include a thin epitaxial layerover a thicker substrate. Substantially all of the detection occurs inthe epitaxial layer, which provides the detection volume. One problemwith electron detectors is that electrons which are backscattered fromthe substrate below the detector volume can return into the detectorvolume, randomly increasing the signal and spreading the signal overmultiple pixels. Performance of the CMOS MAPS can be improved ifelectron backscattering could be prevented. A known method of reducingbackscattering is to thin the substrate below the detector volume, whichis referred to as backthinning. High energy electrons are then morelikely to pass completely through the thinned substrate withoutbackscattering. Ninety percent of the silicon substrate of a CMOS APSdoes not contribute to the performance of the detector but the substratedoes contribute to backscatter which reduces signal to noise ratio andblurs the image.

FIG. 1 shows Monte Carlo simulations of scattering of a 300 keV primaryelectron 102 in a 300 μm thick silicon sensor 104. The traces 106represent different possible electron trajectories as determined by thesimulation. The trajectory that an individual electron follows isdetermined by chance. Only a few of the many possible trajectories thatthe electron could follow are shown. A line 108 is drawn about 35 μmbelow the top surface, the depth to which a typical sensor may bebackthinned. The sensitive volume of the sensor is the top layer 110,approximately 5 μm to 20 μm thick.

Traces 106 show the electron trajectories when a sensor is notbackthinned. Traces 116 show that some of the electrons being scatteredwithin sensor 104 back into the sensitive top layer 110. Suchbackscattered electrons degrade the resolution of the sensor byproducing extraneous signals, many of which are away from the impactpoint of the primary electron 102. With the sensor backthinned to line108, relatively few electrons are backscattered within the thinnedsubstrate to the sensitive top layer 110, so resolution of the sensor isimproved. In a thick sensor, substantial signal could be generated at alarge distance from the impact point of the primary electron 102. A thinsensor can therefore greatly improve the detector performance.

SUMMARY OF THE INVENTION

An object of the invention is to provide a camera for improved electrondetection.

Embodiments of this invention provide improved camera performance byreducing the effect of backscattered electrons. Electrons that passthrough a thin sensor may still have sufficient energy to be scatteredfrom material below the sensor back into the sensor and degrade thedetector signal. Some embodiments remove some or all of the materialbelow a thin detector substrate to prevent scattering electrons backinto the detector volume. Some embodiments provide a structure below thesensor that prevents electrons from backscattering into the detectionvolume. Such structures can prevent backscattering into the detectionvolume by incorporating a geometry that scatters electrons in adirection that prevents them reaching the sensor; by having a lowbackscattering coefficient; or by a combination of geometry and materialproperties.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a Monte Carlo simulation of 300 keV electron tracks in asilicon electron detector.

FIG. 2 shows a few possible trajectories of electrons passing through asensor and backscattering from a material relatively near the bottom ofthe sensor.

FIG. 3 shows a few possible trajectories of electrons passing through asensor and backscattering from a material relatively far below thebottom of the sensor.

FIG. 4 shows an embodiment of the invention in which the geometry of thecamera bottom prevents backscattering to the detector volume.

FIG. 5 shows another embodiment of the invention in which the geometryof the camera bottom prevents backscattering to the detector volume.

FIG. 6 shows another embodiment of the invention in which the geometryof the backside of the sensor prevents backscattering to the detectorvolume.

FIG. 7 shows another embodiment of the invention in which a material ofthe backside of the sensor prevents backscattering to the detectorvolume.

FIG. 8 shows another embodiment of the invention, similar to acombination of the structures shown in FIGS. 5 and 6, in which thegeometry of the camera bottom prevents backscattering to the detectorvolume.

FIGS. 9A and 9B show other embodiments of the invention in which thegeometry of the camera bottom prevents backscattering to the detectorvolume.

FIG. 10 shows a transmission electron microscope that incorporates anembodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Electrons are backscattered not only by the detector substrate, but alsoby materials below the detector substrate. The term “below the sensor”is used herein to mean “downstream” or further along in the direction ofelectron motion from the sensor, regardless of whether the electronsmove vertically or horizontally in the camera. The primary beam ofhigh-energy electrons will pass through a thin sensor and may scatter onmaterial below the sensor back into the sensor and degrade the detectorresolution. As the sensor is made thinner to reduce scattering in thesensor substrate, more electrons pass through the sensor with higherenergies and the problem of backscattering from below the substrate isexacerbated. Various embodiments of the invention prevent backscatteringof electrons from structures below a sensor. “Prevent backscattering” isused to mean to eliminate or to greatly reducing the amount ofbackscattering, and does not require that there be no possibility ofbackscattering. A “thin” sensor means one in which a significantpercentage, that is, greater than ten percent, of the incoming electronspass through the sample and exit the back side. Whether a sensor is“thin” in a particular application depends in part on the energy of theincoming electrons and the material of which the sensor is made. Atypical silicon sensor for a TEM has a pixel size between about 5 μm and50 μm. The sensor thickness includes the thickness of anelectron-sensitive detector volume and the thickness of a substratevolume that provides mechanical support for the detector volume. Asensor that is thinner than about 100 μm is a “thin” sensor in mostapplications, although preferred sensors are less than about 50 μm, lessthan about 35 μm, or less than about 25 μm. In theory, the sensorsubstrate volume could be complete removed, leaving only a detectorvolume between 5 μm and 20 μm thick.

FIG. 2 shows the effect of a material in the camera bottom 202 that isbelow a backthinned detector 204 having a substrate 206 and a sensitivedetector volume 208. FIG. 2 shows a few of the possible trajectories 220that an electron might take after passing through the backthinned sensor204. An electron following one of the trajectories 220 can bebackscattered by the camera bottom 202 and might take one of thetrajectories 222 and return and strike the detector 204. If thereturning electron has sufficient energy to reach the detector volume208, it can cause an erroneous signal. It will be understood that thepath of any individual electron is impossible to predict and thetrajectories 220 and 222 shown represent only a few of the possiblepaths. In accordance with a preferred embodiment of the invention, thevolume below the detector is either empty of material or any materialpresent is configured to reduce backscattering, either by its geometryor by its composition.

FIG. 3 shows that if a camera 300 has no material sufficiently closebelow sensor 204, most electrons that pass through the sensor will notbe backscattered into the sensor 204. FIG. 3 shows three exampletrajectories 302 of an electron 210 that passes through sensor 204, andis then scattered from a material 304 at a relatively large distance 305below the bottom of sensor 204. Trajectories 306 show a few of thepossible trajectories after scattering. Electrons are scattered atangles between zero and ninety degrees. As the size of the gap betweenthe camera floor 304 and the sensor 204 increases, the geometry makes itless likely that the backscattered electron will return and strike thedetector 204. For example, backscattered electrons followingtrajectories 306 would not return and strike detector 204, as didelectrons following trajectories 222 of FIG. 2. In some preferredembodiments, the material below the sensor is positioned at a distanceof more than four times the largest linear surface dimension, such aslength, width, or diameter, of the sensor to substantially reducebackscattering of electrons into the sensor 204. For example, in arectangular sensor, the distance between the sensor bottom and thenearest material is preferably more than four times the longer of thesensor length or width. In other embodiments, a distance of more thantwo times the largest dimension, more than three times the largestdimension, or more than five times the largest dimension is used.

In some preferred embodiments, the bottom of the detector, or thematerial below the detector, can be covered with a material thatmoderates backscatter, such as a material having a low atomic number.For example, beryllium, having an atomic number of four; carbon, havingan atomic number of six; or any material having an atomic number lessthan that of silicon (fourteen), will reduce the backscattering of theelectrons. Compounds, such as hydrocarbons, that have a relatively highhydrogen content have a low average Z and will reduce backscattering.

In some embodiments of the invention, the camera bottom below thedetector provides structural features whose shapes prevent electronsfrom backscattering into the detector. For example, FIG. 4 shows acamera 400 that includes a sensor 402 and a camera bottom 404 that isformed into a series of sloped surfaces 406, such as ridges or pyramids.FIG. 4 shows a few of the possible trajectories 220 of an electron thatpasses through sensor 402. In two of the illustrated trajectories ofFIG. 4, the electron 210 strikes sloped surface 406 and follows atrajectory 408 deeper into one of grooves 410 defined by sloped surfaces406 and away from sensor 402. After multiple collisions, the electronslose energy, are absorbed into the camera bottom 404, and are conductedaway. In another possible trajectory 412, the electron is back scatteredfrom sloped surface 406 away from detector 402. The preferred depth ofgrooves 410 and the angle of sloped surface 406 can be optimized for aparticular camera geometry to maximize attenuation of backscatteringwithin physical constraints of the camera.

FIG. 4 also shows, on the right side of camera 400, that a primaryelectron 420 that impinges near the edge of camera 400 could bescattered by a camera side wall 414 back into sensor 402. Thisbackscatter can be prevented, for example, as shown on the left side ofcamera 400 by a wall structure 430 that includes fins 432 or some otherbarrier. Fins 432 may be horizontal or may extend at an angle,preferably a downward angle, to direct scattered electrons away fromsensor 402. When a primary electron 434 passes through sensor 402 and isscattered off an angled surface 406 toward the side of the camera alonga trajectory 436, the backscattered electron strikes one of fins 432 anddoes not reach the detector 402.

FIG. 5 shows another camera 500 that includes a sensor 502 and a camerabottom 504 that includes multiple vertical fins 506. The vertical fins506 form high aspect ratio channels 508. High aspect ratio in thiscontext and that of other embodiments means an aspect ratio greater than1, more preferably greater than 2 and most preferably greater than 5.The embodiment of FIG. 5 takes advantage of the fact that most electronsare backscattered at an angle, rather than straight back at 180°. FIG. 5shows a primary electron 510 passes through detector 502 and follows oneof trajectories 512. Some of the electrons that strike the flat portions514 of camera bottom 504 are backscattered from camera bottom 504. Whenscattered at an angle they strike the surface of one of the fins 506.After multiple strikes of the fins, the electrons lose energy and areabsorbed. Some electrons that exit sensor 502 at an angle strike thesidewalls of fins 506 and ricochet within channels 508 until they loseenergy. That is, electrons passing the sensor 502 will get trapped inone of the channels 508. The channels 508 could be fabricated usingmaterials commonly used in aerospace construction, such as honeycombaluminum sheets, or with channel plates, such as those used in a channelplate detectors. The channels 508 can be aligned with the pixels ofsensor 502 so that the few electrons backscattered from the tops ofvertical fins 506 will have minimal effect on the detector signal. Thetips of vertical fins 506 can be tapered to a point to further reducebackscattered electrons reaching sensor 502. In some embodiments, fins506 are between about 1 mm and 20 mm high and are spaced apart bybetween 50 μm and 2000 μm. The tops of fins 506 are preferably betweenabout 0 mm and 50 mm below sensor 502. The preferred size of the fins orother backscatter reducing geometric features to reduce backscatteringwill vary with the size of the sensor and the camera geometry. Anychannel structure may reduce backscattering, and optimum dimensions canbe determined experimentally or by using simulations for any particularsensor geometry.

FIG. 6 shows another embodiment of the invention in which a camera 600includes a sensor 604 having fins 606 that form high aspect ratiochannels 608 in the backside of the sensor itself. Rather than uniformlythinning the backside of sensor 604, channels 608 are etched in thebackside, for example, by photolithography. Alternatively, a finned orchannel structure can be bonded or otherwise attached to a thin sensorstructure. FIG. 6 shows that a primary electron 610 that passes throughsensor 604 and follows one of trajectories 614 is scattered at an anglefrom striking camera bottom 616 and strikes fins 606 multiple times. Theelectron loses sufficient energy, similar to the way electrons areabsorbed by the fins 506 of FIG. 5, so that the electron is unlikely toreach the sensitive detector volume 624 in sensor 604. Some electronsare scattered at greater angle within sensor 604 and follow a trajectory626 that strikes a fin 606 before striking camera floor 616. Besidesreducing the number of backscattered electrons, the fins 606 alsoprovides mechanical strength. In some embodiments, a thicker or deepersupportive edge 630 of solid silicon or other material is left aroundthe sensor 604 when the channels 608 are etched to provide additionalmechanical strength. In some embodiment, channels 608 are between about10 μm and 2000 μm deep and are spaced apart by between 10 μm and 2000μm. The bottoms of fins 606 are preferably positioned between about 0 mmand 50 mm above camera bottom 616.

Because of the additional support provided by the fins 606, sensor 604can be made thinner, for example, less than 30 μm, because the thinnedportions have to bridge a much smaller span, in some embodiments, onlytens of microns instead of tens of millimeters. The thinner sensor willfurther reduce backscattering within the substrate. The channels 608 arepreferably aligned with the pixel wells, to minimize the effect on thedetector signal from backscattering from the thicker portions of thedetector substrate at fins 606.

FIG. 7 shows another camera 700 in which resolution is improved byreducing electron backscattering from below the sensor substrate. Thebackside of a sensor 702 is coated with a layer 704 of material thatreduces backscattering. For example, the material of layer 704 couldhave a low backscatter coefficient, which is typical of materials havinglow average atomic numbers, including hydrocarbons such as bees wax, ora sheet of low density material, such as reticulated ventricular carbon(“RVC”). This embodiment also has the advantage of providing structuralsupport for the thin detector 702. Additional supports allow thedetector to be made thinner than would be possible without the support.In one embodiment, a one millimeter layer of RVC reduces the number ofbackscattered electrons reaching the detection volume of detector 702.Material 704 should be vacuum compatible, that is, have a low vaporpressure. A thin layer of a high Z material may be used to cover andprotect a higher vapor pressure material, such as a hydrocarbon, fromthe vacuum. As long as the high Z layer is thin, for example, less than50 μm, the increased backscatter will be within acceptable levels formany applications. Primary electron 720 passes through sensor 702following one of the trajectories 722. With some materials, such ashydrocarbons, as electrons following trajectories 722 pass throughmaterial 704, they lose energy. Electrons that are backscattered bycamera bottom 730 pass again into material 704 where they losesufficient energy so that most do not reach the detector volume 732.

FIG. 8 shows a camera 800 that includes a combination of the embodimentsof FIGS. 5 and 6. The camera of FIG. 8 includes a sensor 604 havingchannels 608. Below sensor 604 is a camera bottom 504 that includesmultiple vertical fins 506 that define channels 508. The camera bottom504 or the sensor 602 could also include within the channels a materialthat reduces backscattered electrons, such as the materials describedwith respect to FIG. 7.

A primary electron 820 passing through the detector 604 and followingone of 822 trajectories ricochets and is absorbed in channels 608 and508, as described with respect to FIGS. 5 and 6.

FIGS. 9A and 9B shows additional embodiments of structures for reducingelectron backscatter downstream from a thin sensor. FIG. 9A shows astructure 900 a that includes angled fins 906 a having between themspaces 908 a. The angle of the fins 906 a is preferably sufficient toprevent electrons from the sensor from directly impacting camera bottom914 a. Electrons that pass through the sensor are scattered from thesides of angled fins 908 a and are effectively trapped in structure 900a. The tops of angled fins 906 a can be angled with respect to thebottom surface of the sensor to prevent backscatter from the tops of thefins. The structure 900 b of FIG. 9B is similar to structure 900 a ofFIG. 9A, but angled fins 906 b vary in height. As in structure 900 a,electrons passing through the sensor become trapped in spaces 908 bbetween angled fins 906 b.

FIG. 10 shows a simplified block diagram of a transmission electronmicroscope 1000 that embodies the invention. Microscope 1000 includes asource of electrons 1002 and a focusing column 1004 that focuses a highenergy beam of electrons onto a sample 1006. Electrons passing throughthe sample 1006 are detected by a thin sensor 1008 of a camera 1010.Structure 1012 improves detector resolution by preventing backscatter ofelectrons back into sensor 1008 from scattering from camera floor 1014.Some of the possible embodiments of structure 1010 are described inFIGS. 3-8 above. There are many known configurations of transmissionelectron microscopes, and the invention can be used on any microscopethat includes a thin sensor. For example, embodiments of the inventioncan be used with a microscope having an energy filter for energy lossspectroscopy or for forming an energy filtered image.

While several embodiments have been separately described, skilledpersons will recognize that the various structures described to reducebackscattering can be combined in various ways to reduce backscatteringwithout departing from the principals of the invention. For example, thecamera bottom surface could comprise an irregular, rough surface toreduce backscattering instead of a regular geometric pattern asdescribed above. Any deviation from a smooth surface will reducebackscattering. By “camera bottom” is meant the portion of the cameradownstream of the sensor, and is not limited to the bottom vacuumchamber wall or any particular component. All dimensions in theembodiments above are provided by way of example, and the dimensions ofthe various structures to prevent backscattering will be varieddepending on the characteristics, such as sensor size, of each camera.While a “backthinned” sensor is referenced in some of the embodimentsabove because of current manufacturing techniques, the invention isapplicable to any thin sensor, and is not limited to a sensor that isoriginally thick and is thinned from the back side. The invention isapplicable not only to CMOS APSs, but to any type of thin sensor, suchas silicon strip detectors, or matrix detectors using avalanche photodiode or other thin sensor currently known or to be developed, includingsemiconductor or non-semiconductor based sensors.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

We claim as follows:
 1. A method of observing a sample in a transmissionelectron microscope, comprising: directing a beam of high energyelectrons towards a sample; detecting electrons that pass through thesample using a thin sensor comprising an electron-sensitive detectorvolume through which a substantial number of electrons pass; providingbelow the thin sensor a structure configured to reduce a backscatteringof electrons toward the thin sensor such that a resolution of the thinsensor is improved by a reduction in noise from backscattered electrons;and using the structure to prevent electrons that have passed throughthe thin sensor and into an empty space below the sensor from scatteringback into the thin sensor and producing erroneous signals.
 2. The methodof claim 1 in which providing the structure includes providing astructure comprising multiple high aspect ratio channels.
 3. The methodof claim 2, in which the multiple high aspect ratio channels opentowards the thin sensor.
 4. The method of claim 2 in which the multiplehigh aspect ratio channels open away from the thin sensor.
 5. The methodof claim 1 in which providing below the structure includes providing astructure having multiple sloped surfaces to deflect electrons toprevent electrons from reaching the electron-sensitive detector volume.6. The method of claim 1 in which providing the structure includesproviding a structure having a low backscattering coefficient to preventelectrons from reaching the electron-sensitive detector volume.
 7. Themethod of claim 1 in which providing the structure includes providing astructure that includes the empty space below the thin sensor, the emptyspace having a thickness of at least four times a size of the thinsensor.
 8. The method of claim 1 in which the structure comprises amaterial having an atomic number or an average atomic number less thanthe atomic number of silicon to reduce the backscattering of electrons.9. The method of claim 8 in which the structure comprises beryllium orcarbon.
 10. The method of claim 1 in which the thin sensor is thinnerthan about 50 μm.
 11. The method of claim 1 in which theelectron-sensitive volume comprises a thickness in a range of from 5 μmto 20 μm.
 12. A method of observing a sample in a transmission electronmicroscope, comprising: directing a beam of high energy electronstowards a sample; detecting electrons that pass through the sample usinga thin sensor comprising an electron-sensitive detector volume throughwhich a substantial number of electrons pass; and providing below thethin sensor a structure comprising a hydrocarbon that prevents electronsthat have passed through the thin sensor from scattering back into thethin sensor and producing erroneous signals.
 13. The method of claim 12in which the structure has a low backscattering coefficient to preventelectrons from reaching the electron-sensitive detector volume.
 14. Amethod of observing a sample in a transmission electron microscope,comprising: directing a beam of high energy electrons towards a sample;detecting electrons that pass through the sample using a thin sensorcomprising an electron-sensitive detector volume through which asubstantial number of electrons pass; providing below the thin sensor astructure comprising sloped fins having different heights, the structureconfigured to reduce a backscattering of electrons toward the thinsensor such that a resolution of the thin sensor is improved by areduction in noise from backscattered electrons; and using the structureto prevent electrons that have passed through the thin sensor fromscattering back into the thin sensor and producing erroneous signals.15. A method of observing a sample in a transmission electronmicroscope, comprising: directing a beam of high energy electronstowards a sample; detecting electrons that pass through the sample usinga thin sensor comprising an electron-sensitive detector volume throughwhich a substantial number of electrons pass; providing below the thinsensor a structure comprising vertical fins, the structure configured toreduce a backscattering of electrons toward the thin sensor such that aresolution of the thin sensor is improved by a reduction in noise frombackscattered electrons; and using the structure to prevent electronsthat have passed through the thin sensor from scattering back into thethin sensor and producing erroneous signals.
 16. The method of claim 15in which tips of the vertical fins are tapered to a point.
 17. Themethod of claim 15 in which the vertical fins have heights in a range offrom about 1 mm to about 20 mm and are spaced apart at distances in arange of from about 50 μm to about 2000 μm.
 18. The method of claim 15in which tops of the vertical fins are located at distances below thethin sensor in a range of from about 0 mm to about 50 mm.
 19. A methodof observing a sample in a transmission electron microscope, comprising:directing a beam of high energy electrons towards a sample; detectingelectrons that pass through the sample using a thin sensor comprising anelectron-sensitive detector volume through which a substantial number ofelectrons pass; providing below the thin sensor a structure comprisingmultiple channels having aspect ratios greater than 1 and configured toreduce a backscattering of electrons toward the thin sensor such that aresolution of the thin sensor is improved by a reduction in noise frombackscattered electrons; and using the structure to prevent electronsthat have passed through the thin sensor from scattering back into thethin sensor and producing erroneous signals.
 20. The method of claim 19in which the multiple channels have aspect ratios greater than
 2. 21.The method of claim 20 in which the multiple channels have aspect ratiosgreater than 5.